Should I install second Antivirus or Firewall program improve my systems' security?

NO! More than one Anti-Virus program will overburden your PC by consuming its computing power and is not likely to do anything but require a good amount of time trying fix your system. More than one firewall program will most likely completely block your Internet access altogether. Choose one good Anti-Virus program such as AVAST!'s Free Home or Pro Edition, or AVG version 8, and if you are using Windows XP - it has a pretty good firewall already built in - just disable options like "remote support" and other port openings that could lead to vulnerabilities.